runtime: improve AddCleanup documentation
Steer people from SetFinalizer to AddCleanup. Address some of the
*non*-constraints on AddCleanup. Add some of the subtlety from the
SetFinalizer documentation to the AddCleanup documentation.
Updates #67535.
Updates #70425.
Change-Id: I8d13b756ca866051b8a5c19327fd5a76f5e0f3d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/634318
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Austin Clements <austin@google.com>